1. Field of the Invention
The present invention relates to a metal object forming method which is advantageously used for producing a metal housing of portable electronic devices such as notebook computers and cell phones. The present invention also relates to a metal housing produced by the method.
2. Description of the Related Art
In portable electronic devices including notebook computers, cell phones, etc., the housing is often made of light metal (e.g., magnesium alloy or aluminum alloy) for attaining weight reduction and good heat dissipation. Such a metal housing, which often includes thin walls and complex shapes, can be produced by a die-casting technique. As is known, in die-casting, use is made of dies, or molds, that are designed to define a cavity corresponding to the desired shape. Molten metal is injected into the die cavity, in which the supplied metal hardens. Then, the die is opened, and the finished casting is ejected. A die-casting technique is disclosed in JP-A-9(1997)-272945, for example.
The conventional die-casting technique has found disadvantageous in the following respects.
In general, the molten metal injected into the die cavity will cool due to the heat conduction from the molten metal to the dies. When the cavity includes a relatively large portion and a relatively narrow portion, the molten metal tends to cool more sharply in the narrow portion than in the spacious portion. Unfavorably, the flowability of the molten metal becomes poorer as the temperature of the metal lowers. Accordingly, the molten metal in the narrow portion may harden before it can reach the end of the cavity. By a conventional die-casting technique, such a defect often results when the narrow portion of the die cavity is no greater than 1.5 mm.
Turning to another aspect of the conventional technique, a mold-releasing agent is often used for performing die-casting so that the resultant casting is readily separated from the cavity-defining surfaces of the die. JP-A-5(1993)-92232, for example, discloses a mold-releasing agent containing powered boron nitride, silicon nitride or mica. According to the teaching of this JP document, the mold-releasing agent is applied to the cavity-defining surfaces of the die, and then molten metal is injected into the closed dies. In this manner, the injected metal is spaced from the cavity-defining surfaces of the die by the particles contained in the mold-releasing agent. Thus, the resultant casting can be readily ejected from the dies. It should be noted, however, that this conventional mold-releasing agent can work for facilitating the separation of the casting from the dies, but not for improving the flowability of the molten metal.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a metal object forming method whereby a metal object with thin walls can be produced properly without suffering a defect resulting from poor flowability of the molten metal. Another object of the present invention is to provide a housing of an electronic device that is produced by such a method.
According to a first aspect of the present invention, there is provided a metal object forming method. At a preliminary step, a molding die is supplied with flowability-improving material which melts into molten metal and causes the freezing point depression of the molten metal. At a subsequent injecting step, molten metal is supplied into the die for producing a casting.
Preferably, the flowability-improving material may include metal particles contained in lubricant. At the preliminary step, this lubricant is applied to a cavity-defining surface of the die. At the injecting step, the molten metal is supplied at a temperature high enough to melt at least part of the metal particles.
Preferably, the metal particles may be coated with thermoplastic resin such as olefin resin, acrylic resin or styrene resin.
Preferably, the particles may have a diameter of 1xcx9c100 xcexcm.
Preferably, the lubricant may contain 5xcx9c30 wt % of metal particles.
Preferably, the flowability-improving material may include a metal plate. At the preliminary step, the metal plate is disposed on a cavity-defining surface of the die. At the injecting step, the molten metal is supplied at a temperature high enough to melt at least part of the metal plate.
Preferably, the flowability-improving material may include zinc or zinc-based alloy, while the molten metal may include magnesium or magnesium-based alloy.
Preferably, the zinc-based alloy may contain 60xcx9c95 wt % of zinc and 5xcx9c40 wt % of tin.
According to a second aspect of the present invention, there is provided a housing of an electronic device, wherein the housing is produced by the above-mentioned method.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.